The invention is concerned with the oligomerization of supercritical ethene.
This application claims the benefit of European Application No. 16173939.6 filed on Jun. 10, 2016, the disclosure of which is expressly incorporated herein by reference.
Hydrocarbons are chemical compounds which consist exclusively of carbon and hydrogen. Alkenes (synonym: olefins) are hydrocarbons which have one C═C double bond in the molecule. Alkanes (synonym: paraffins), on the other hand, are hydrocarbons which have only single bonds. They are therefore also referred to as saturated.
In organic chemistry, hydrocarbons are frequently designated according to the number of carbon atoms which they have per molecule, in that the respective class of substances is preceded by the prefix Cn. “n” is the respective number of carbon atoms in a molecule. Thus, C4 olefins are substances from the class of alkenes having four carbon atoms. C8 olefins correspondingly have eight carbon atoms per molecule. Where the prefix Cn+ is used hereinafter, it refers to a class of substances which have more than n carbon atoms per molecule. A C4+ olefin accordingly has at least five carbon atoms.
The simplest olefin is ethene (ethylene). It has two carbon atoms. Ethene is an important commodity chemical and is therefore prepared in large quantities. This is usually effected by cracking of naphtha. In addition, it can be obtained by dehydrogenation of ethane, which in turn is a constituent of natural gas. Owing to the increasing exploitation of unconventional sources of natural gas and decreasing recovery of petroleum, the proportion of ethene based on natural gas is steadily increasing. The physical properties of ethene and the preparation thereof are described in:    Zimmermann, Heinz and Walzl, Roland: Ethylene. Ullmann's Encyclopedia of Industrial Chemistry (2009).
Oligomerization is understood to mean the reaction of hydrocarbons with themselves, forming correspondingly longer-chain hydrocarbons, called the oligomers. Olefins having from two to eight carbon atoms can be oligomerized very readily.
Thus, for example, an olefin having six carbon atoms (hexene) can be formed by oligomerization of two olefins having three carbon atoms. The oligomerization of two molecules with one another is also referred to as dimerization. If, in contrast, three olefins having three carbon atoms are joined to one another (trimerization), the result is an olefin having nine carbon atoms. If n-butenes—i.e. olefins having four carbon atoms—are subjected to an oligomerization, the result is essentially olefins having eight carbon atoms (more specifically: dibutene), and additionally olefins having twelve carbon atoms (C12 olefins, “tributene”) and, to a smaller extent, olefins having more than twelve carbon atoms (C12+ olefins).
Depending on the number of carbon atoms and what is called the degree of branching, the oligomers are used for the production of plasticizer esters or detergents or as a fuel additive:    Friedlander, Ward, Obenaus, Nierlich, Neumeister: Make plasticizer olefins via n-butene dimerization. Hydrocarbon Processing, February 1986, pages 31 to 33.    F. Nierlich: Oligomerize for better gasoline. Hydrocarbon Processing, February 1992, pages 45 to 46.
The oligomerization of ethene only becomes industrially practicable through use of catalysts. Industrial processes for oligomerization of ethene can be roughly divided into homogeneously catalysed processes and heterogeneously catalysed processes.
In homogeneously catalysed processes, the catalyst is dissolved in the reaction mixture. The reaction is effected in the liquid phase, in which the dissolved catalyst is also present. An advantage of homogeneous catalysis is the high efficiency of the reaction; a disadvantage is that the dissolved catalyst has to be separated from the reaction mixture in a costly and inconvenient manner. If the catalyst is very inexpensive (for example triethylaluminium) and catalyst residues do not impermissibly soil the oligomerizate, it is possible to dispense with a removal. Examples of homogeneously catalysed oligomerization of ethene are disclosed by WO2005/123633 and US2013/0066128.
In heterogeneously catalysed processes, the catalyst is in the solid state in the reactor. The ethene that flows through the reactor comes into contact with the solid catalyst. The oligomerizate is drawn off from the reactor and the solid catalyst remains in the reactor. An advantage of the heterogeneously catalysed processes is that the costly and inconvenient removal of catalyst is dispensed with and the reaction product is not contaminated by breakdown products of the catalyst. A disadvantage is that the catalyst is deactivated with time, especially in that polyethylene precipitates on the catalyst and blocks the catalytically active sites. The service life of the catalyst is therefore limited. A deactivated catalyst has to be either exchanged or regenerated.
Heterogeneous oligomerization can be effected either in the liquid phase or in the gaseous phase. Liquid phase oligomerization has the advantage that the reaction mixture has a greater density and hence the apparatus volume is more efficiently exploited. An advantage of the gas phase oligomerization of ethene is the lower pressures which make the apparatus less expensive and easier to control.
In addition, there are first approaches to oligomerizing ethene in the supercritical state:
WO1995/14647A1 claims a process for oligomerizing unbranched C2 to C6 olefins to dimers, trimers and tetramers by means of a fixed bed catalyst, in which the oligomerization is conducted in a reaction zone at supercritical temperature and supercritical pressure of the olefins used and no additional solvents that are not in the supercritical state in the reaction zone are used. The catalyst used comprises titanium dioxide, aluminium oxide, nickel oxide and silicon dioxide, and also alkali metal oxide resulting from the preparation. This catalyst is said to be of excellent suitability for oligomerization of butenes in butene/butane mixtures. However, the suitability thereof for the oligomerization of ethene is not discussed in WO1995/14647A1. There is a lack of specific details relating to the solvents used (substance, concentration, thermodynamic state). The emphasis of the contents of this publication is more on the production of the fixed bed catalyst than on the reaction regime.
WO2010/117539A2 discloses a process for oligomerizing ethene which occurs in a highly contaminated feed mixture. The feed mixture originates from a fluid-catalytic naphtha cracker (FCC) and comprises, as main constituents, methane and/or ethane, greater amounts of hydrogen and nitrogen, and carbon monoxide, carbon dioxide and hydrogen sulphide as impurities. It is mentioned in passing that the pressure can be conducted above the critical pressure of pure ethene; no specific details at all are given with regard to the critical temperature of ethene. The reaction is said to proceed essentially in the gas phase. It is a declared aim of WO2010/117539A2 to convert contaminated ethene-containing offgases to liquid fuels which can be added to the diesel or gasoline pool. Since the calorific value of the fuels rises with the number of carbon atoms in the hydrocarbons present therein, it makes sense to optimize such a process in the direction of production of longer-chain oligomers. In fact, the oligomerizate described in WO2010/117539A2 contains a large amount of olefins having ten or more carbon atoms.
By contrast, the processes optimized to the production of olefins having four to eight carbon atoms from ethene are those that are set out in some applications that were yet to be published at the priority date of this application. In said applications, ethene is oligomerized in the presence of at least one inert solvent. For instance, n-hexane is used as solvent in U.S. Ser. No. 15/000,807 or in EP 16151490.6, whereas propane or isobutane is preferred as solvent in U.S. Ser. No. 15/000,837 or in EP 15151624.2. In all cases, both the ethene and the solvent are in the liquid state.
Likewise still unpublished at present is European application 16158044.4 or U.S. Ser. No. 15/056,147, which describes the in situ regeneration of a solid catalyst used in ethene oligomerization. The regeneration is effected with the aid of a liquid purge medium which serves as solvent for the ethene in oligomerization operation. Purge media or solvents proposed are propane, isobutane, pentane, cyclopentane, hexane, cyclohexane, heptane or cycloheptane. Both in oligomerization operation and in regeneration operation, the solvent or purge medium is in the liquid state.
With regard to this prior art, the problem underlying the invention is that of specifying a process for oligomerization of ethene which enables a distinct rise in the space-time yield. In addition, a high selectivity in the direction of the dimers, trimers and tetramers of ethene is to be achieved.
Olefins having 10 or more carbon atoms (C10+ olefins), by contrast, are barely to be formed. Furthermore, the process is to be efficient; more particularly, the apparatus volume is to be exploited to the best possible degree. A further important aim is the prolonging of the service life of the catalyst.